The present invention relates to a device for chucking a workpiece to be machined.
Conventionally, for chucking a workpiece to be machined, as shown in Japanese Patent Laid-Open Publication No. HEI 5-104312, first, fluid is fed under pressure to behind a piston in a hollow chamber through a passage so that the piston is slid. With this sliding of the piston, a central portion of a diaphragm is pushed forward, by which chucking claws are opened wide.
Next, a workpiece is inserted inside the wide-open chucking claws, and then the fluid is stopped from being fed.
Then, the piston is returned by restoring force of the diaphragm, so that the chucking claws are contracted. Thus, the work is chucked.
Further, the fluid is fed under pressure to the front side of the piston in the hollow chamber through the passage, so that the piston is slid backward, while the central portion of the diaphragm is pulled backward. In this way, the chucking claws are contracted toward the center.
By doing so, a greater chucking force can be obtained.
With such a chucking device, which is so arranged that the piston and the diaphragm connected to the piston are pulled up together by fluid pressure that acts on the pressure-receiving surface, there has been a disadvantage that a great clamping force could not be obtained.
Accordingly, as shown in FIG. 8, there has been provided a chucking device in which a first piston 3 and a second piston 4 connected to a diaphragm 2 are built in a hollow chamber 1, and fluid is fed under pressure to the front side of the first piston 3 and the second piston 4 in the hollow chamber 1 through a passage 5, so that greater power is obtained (double piston system).
In addition, more than two pistons may be arrayed so that even greater power can be obtained.
In this case, a considerably large weight increase is involved due to the double piston system such that the device would be unsuitable for high-speed revolution, and the size is also increased, thereby posing disadvantages.